Resolution of chiral amines

ABSTRACT

Chiral amines are resolved by selectively reacting an enantiomer of the amine with an alkyl ester in the presence of an enantioselective lipase enzyme to produce an amide of that enantiomer and separating it from the unreacted enantiomer, the alkyl group of the ester being an isoalkyl group. Isobutyl and especially isopropyl groups are preferred.

This application is the national phase of international application PCT/C7B98/03769 filed Dec. 9, 1998 which designated the U.S.

THIS INVENTION relates to the resolution of chiral amines.

It is known for example from WO95/08636, Kitaguchi et al (J.Am. Chem. Soc., 1989, 111, 3094-3095), Gotor et al (J.Chem. Soc. Perkin Trans. 1, 1993, 2453-2456), Ohmer et al (Enzyme and Microbial Technology, 1996, 19, 328-331) and Sanchez et al (Tetrahedron Asymmetry, 1997, 8, 37-40) and Chiou et al, Bio-organic & Medicinal Chemistry Letters Vol. 7, No 4, pp 433-436 (1977 to resolve racemic amines by acylating one enantiomer by reaction with an alkyl ester in the presence of an enantioselective enzyme as catalyst However, the results obtained have in many cases been disappointing.

We have found that such reactions of attractive stereospecificity occur if the alkyl group of the ester is an isoalkyl group preferably an isopropyl group.

The invention therefore comprises a process of resolution of chiral amines which comprises selectively reacting one enantiomer of the amine with an alkyl ester in the presence of a enantioselective lipase enzyme to produce an amide of one enantiomer and separating it from an unreacted enantiomer optionally after further reaction charactersed in that the alkyl group of the ester is an isoalkyl group and preferably an isopropyl group. A lipase is an enzyme capable of catalysing the esterification of aliphatic adds with glycerol and the hydrolysis of esters of glycerol and aliphatic adds.

Either or both enantiomers may be recovered. The untreated enantiomer may be recovered as such. The reacted enantiomer may be converted to the original amine enantiomer suitably by hydrolysis. It may of course be utilised as the amide if desired. Suitably such hydrolysis may be carried out using as catalyst an amidase of the same stereospecificity and/or hydrolysing any unwanted stereoisomer present using an amidase of opposite stereospecificity, separating the unwanted amide and hydrolysing that, thus providing a second stage of resolution and enhancing the enantiomeric excess of the product, but if the first stage provides sufficient specificity a non selective hydrolysis may be employed.

The acid component of the ester may have 1 to 10 for example 1 to 5 carbon atoms. It is preferably of formula RCOOH in which R is a hydrocarbyl group, for example an aryl group such as a phenyl, naphthyl or benzyl group, an alkyl or cydoalkyl group or a chloroor bromo substituted derivative thereof, the substitution being preferably on a carbon atom adjacent to the C═O group or one next to it It may suitably be an unsubstituted alkyl group suitably having 1 to 4 carbon atoms as these are often of moderate cost tend not to be involved in unwanted side reactions and tend not to be aggressive to metal reaction vessels.

The process is suitable for the resolution of primary and secondary amines for example amines of formula

in which R¹ and R² are alkyl, cycloalkyl, alkenyl or alkynyl, or an aryl group or such a group, which is substituted with for example NO₂, SO₃H, COOR⁴, Cl, Br, F, I, OH, SO, SO₂, CN, alkoxy and in the case of aryl substitution NH₂ in which R¹ and R² are different and R³ is H, alkyl, cycloalkyl, alkenyl, alkynyl, or an aryl group or such a group which is substituted with for example, NO₂, SO₃H, COOH, Cl, Br, F, I, OH, SO, SO₂, CN and R⁴ is alkyl, cycloalkyl, alkenyl, alkynyl or an aryl group optionally substituted as described above. The process is suitable for the resolution of amino acids and their esters.

The amine preferably has the formula

in which R⁴ is an alkyl group having for example1 to 12 and preferably 1 to 6 carbon atoms and R⁵ is an aryl preferably a naphthyl group, an alkyl group or cycloalkyl group in each case optionally substituted by one or more alkoxy, hydroxy, halogen and/or —CN group or in the case of aryl groups, amine groups, groups which preferably have at most 12 and more preferably at most 6 carbon atoms in total in all of the said substituents.

The amount of lipase present is preferably 10 to 50% by weight of the amine. The lipase is preferably supported on a solid support to enable it to be removed mechanically, for example by filtration or centrifugation, after reaction.

Separation of the amide of the reacted amine from unreacted amine may be accomplished by known methods, for example, distillation or crystallisation.

The reaction may be carried out in the presence of a solvent which may be the ester, an ether (for example methyl tert butyl ether, dimethoxy ethane or tetrahydrofuran) or a hydrocarbon, for example toluene or an alkane or cycloalkane having 5 to 10 carbon atoms or a halogenated hydrocarbon solvent. It is preferably free from —OH and NH₂ groups.

The reaction may be carried out at 20-60° C. for example at 20-40° C. At least one mole of the ester should be provided per two moles of the amine so as to permit stoichiometric reaction of an enantiomer, but it is preferred that an excess be provided. The excess should normally be sufficient to provide preferably at least 90% and more preferably at least 95% for example 99% reaction of the most reactive enantiomer. In judging the appropriate excess, conditions should not be such as to cause unacceptable conversion of the less reactive enantiomer, and if a very high selectivity for the more reactive enantiomer is needed it may be preferred to convert only part thereof, thus requiring little or no excess; indeed operation with less than the stoichiometrc amount may be desirable in some cases.

The step of converting the amine to the amide is preferably carried out in the substantial absence of water and other hydroxy compounds.

EXAMPLE 1

2-Amino-3, 3-dimethylbutane (125 mg) was added to 3 ml of acyl donor (e.g. ethyl acetate or isopropyl acetate) and incubated at 28° C. in the presence of 60 mg of immobilised lipase from Candida antarctica (NOVO SP 435). The extent of reaction was followed by quantitative gas chromatography on a Perkin Elmer 8500 system fitted with a J&W 30 m×0.32 mm fused silica capilliary GC The gas chromatograph was operated with helium carrier gas at 5.5×104 Pa (8 psi) using a temperature gradient starting at 80° C. and rising to 200° C. at a rate of 20° min⁻¹ followed by 6 minutes held at 200° C. Under such conditions 2-amino-3, 3-dimethylbutane was eluted with a retention time of 3.0 minutes and the corresponding acetamide at 6.1 minutes. The enantiomeric purity of unreacted 2-amino-3, dimethylbutane (derivatised as the N-butyramide) and the acetamide product of the aminolysis reaction was determined by chiral phase gas chromatography using a 25 m×0.32 mm Chrompack CP-Chirasil-Dex CB column. The gas chromatograph was operated with a helium carrier gas at 5.5×104 Pa (8 psi) and at 20° C. Under such conditions the two enantiomers of the acetamide derivative of 2-amino-3, 3-dimethylbutane were eluted with retention times of 4.78 minutes (S) and 4.95 minutes (R) and the two enantiomers of the butyramide derivative 8.38 minutes (S) and 8.51 minutes (R). The results are summarised in Table 1.

TABLE 1 Effect of structural changes to the alcohol component of the acyl donor on the enantioselectivity of the resolution of 2-amino-3,3-dimethylbutane by Candida antarctica lipase. E.e. unreacted Reac- amine (as E.e. Acyl tion Conversion butyramide) product Enantiomeric donor time (h) (%) (%) (%) ratio (E) Ethyl 216 39.2 50 78 13 acetate Isopropyl 216 46.5 83 95 104 acetate

EXAMPLE 2

1-(1-Naphthyl) ethylamine (100 mg) was added to 5 ml of acyl donor e.g. ethyl acetate or isopropyl acetate) and incubated at 28° C. in the presence of 50 mg of immobilised Candida antarctica lipase (NOVO SP 435). The extent of conversion was determined from measurements of the enantiomeric excess of the unreacted amine (derivatised as its butyramide) and product acetamide using the mathematical expression described by Chen et. al. (J. Am. Chem. Soc., 1982, Vol 104, pp 7294-7299). Enantiomeric excess measurements were made by chiral phase HPLC on a Hewlett Packard HP 1050 system fitted with a 250 mm×4.6 mm Daicel Chiralcel OD column. The column was eluted isocratically with a mixture of 92.5% hexane and 7.5% ethanol in 1 ml min⁻¹. Compounds were detected by UV absorbance at 254 mm. The retention times of the unreacted amine (derivatised as its butyramide) were 7.65 minutes (R) and 15.2 minutes (S) and the product acetamide 8.9 minutes (R) and 17.9 minutes (S). The results are summarised in Table 2.

TABLE 2 Effect of structural changes to the alcohol component of the acyl donor on the enantioselectivity of the resolution of 1-(1-naphthyl) ethylamine by Candida antarctica lipase. E.e. unreacted Reac- amine (as E.e. Acyl tion Conversion butyramide) product Enantiomeric donor time (h) (%) (%) (%) ratio (E) Ethyl 70 52.5 66 60 8 acetate Isopropyl 66 48.9 88 92 70 acetate

EXAMPLE 3

Racemic 1-(1-naphthyl)ethylamine (100 mg) was added to 5 ml of acyl donor (see Table 3) and incubated at ambient temperature in the presence of 20 mg of Chirazyme L2 (immobilised Candida antarctica lipase). At intervals 0.5 ml samples were removed and diluted to 1 ml with a solution of hexanelethanol (92.5:7.5). The unreacted amine was converted to its corresponding butyramide by the addition of 10 μl of butyric anhydride. Each sample was analysed by chiral phase HPLC on a Hewlett Packard HP1050 system using a Daicel Chiralcel OD analytical column (250 mm×4.6 mm) eluted with hexanelethanol (92.5:7.5) at a flow rate of 1 ml min⁻¹. Compounds were detected by UV absorbance at 254nm. The retention times of the two enantiomers of the unreacted amine (derivatised as its butyramide) were 7.9 minutes (R) and 15.0 minutes (S) and the product acetamide 9.3 minutes (R) and 17.9 minutes (S). The extent of conversion and enantiomeric ratio was determined from measurements of the enantiomeric excess of unreacted amine and product acetamide using the mathematical expression described by Chen et. al. (J. Am. Chem. Soc., 1982, Vol 104, pp 7294-7299). The results are summarised in Table 3.

TABLE 3 Effect of structural changes to the alcohol component of the acyl donor on the enantiospecificity of the resolution of 1-(1-naphthyl)ethylamine by Chirazyme L2 lipase. E.e. of E.e. of product Time Conversion unreacted acetamide Enantiomeric Acyl donor (h) (%) amine (%) (%) ratio (E) Methyl 120 30 6 13 1 acetate Ethyl 120 43 59 79 15 acetate n-Propyl 120 40 54 81 16 acetate n-Butyl 120 32 40 77 18 acetate Isopropyl 72 45 78 95 100 acetate Isobutyl 72 50 86 88 37 acetate Isoamyl 72 44 68 88 28 acetate

EXAMPLE 4

Racemic 1,2,3,4-tetrahydro1-naphthylamine (100 mg) was added to 5 ml of acyl donor (see Table 4) and incubated at ambient temperature in the presence of 20 mg of Chirazyme L2 (immobilised Candida antarctica lipase). At intervals 0.5 ml samples were removed and diluted to 1 ml with dichloromethane. The unreacted amine was converted to its corresponding butyramide by the addition of 10 μl of butyric anhydride. Each sample was analysed by chiral phase GC on a Perkin Elmer 8700 system using a Chrompack CP-Chirasil-Dex CB column (25 m×0.32 mm). The gas chromatograph was operated isothermally at 175° C. with helium carrier gas at 5.5×104 Pa (8 psi) Compounds were detected by flame ionisation. The retention times of the two enantiomers of the unreacted amine (derivatised as its butyramide) were 23.8 minutes (S) and 25.8 minutes (R) and the product acetamide 14.4 minutes (S) and 15.9 minutes (R). The extent of conversion and enantiomeric ratio was determined from measurements of the enantiomeric excess of unreacted amine and product acetamide using the mathematical expression described by Chen et. al (J. Am. Chem. Soc., 1982, Vol 104, pp 7294-7299). The results are summarised in Table 4.

TABLE 4 Effect of structural changes to the alcohol component of the acyl donor on the enantiospecificity of the resolution of 1,2,3,4-tetrahydro- 1-naphthylamine by Chirazyme L2 lipase. E.e. of E.e. of product Time Conversion unreacted acetamide Enantiomeric Acyl donor (h) (%) amine (%) (%) ratio (E) Methyl 72 57 16 12 1 acetate Ethyl 72 54 82 69 14 acetate n-Propyl 72 52 78 74 14 acetate n-Butyl 72 26 14 42 3 acetate Isopropyl 72 50 98 97 458 acetate Isobutyl 72 53 95 83 43 acetate Isoamyl 72 46 70 81 21 acetate

EXAMPLE 5

Racemic 2-amino3,3-dimethylbutane (100 mg) was added to 5 ml of acyl donor (see Table 5) and incubated at ambient temperature in the presence of 40 mg of Chirazyme L2 (immobilised Candida antarctica lipase). At intervals 0.5ml samples were removed and diluted to 1 ml with dichloromethane. The unreacted amine was converted to its corresponding butyramide by the addition of 10 μl of butyric anhydride. Each sample was analysed by chiral phase GC on a Perkin Elmer 8700 system using a Chrompack CP-Chirasil-Dex CB column (25 m×0.32 mm). The chromatograph was operated isothermally at 120° C. with helium carrier gas at 5.5×104 Pa (8 psi). Compounds were detected by flame ionisation. The retention times of the two enantiomers of the unreacted amine (derivatised as its butyramide) were 10.3 minutes (S) and 10.5 minutes (R) and the product acetamide 5.6 minutes (S) and 5.9 minutes (R). The extent of conversion and enantiomeric ratio was determined from measurements of the enantiomeric excess of unreacted amine and product acetamide using the mathematical expression described by Chen et al. (J. Am. Chem. Soc., 1982, Vol 104, pp 7294-7299). The results are summarised in Table 5.

TABLE 5 Effect of structural changes to the alcohol component of the acyl donor on the enantiospecificity of the resolution of 2-amino-3,3-dimethylbutane by Chirazyme L2 lipase. E.e. of E.e. of product Time Conversion unreacted acetamide Enantiomeric Acyl donor (h) (%) amine (%) (%) ratio (E) Methyl 168 29 2 5 1 acetate Ethyl 168 34 36 69 8 acetate n-Propyl 168 26 24 69 7 acetate n-Butyl 168 17 13 62 5 acetate Isopropyl 168 41 67 98 109 acetate Isobutyl 168 33 44 88 27 acetate Isoamyl 168 25 26 79 10 acetate

EXAMPLE 6

Racemic 1-(1-naphthyoethylamine (100 mg) was added to a solution of 4 ml of dimethoxyethane and 1 ml of acyl donor (see Table 6) and incubated at ambient temperature in the presence of 20 mg of Chirazyme L2 (immobilised Candida antarctica lipase). At intervals 0.5 ml samples were removed and diluted to 1 ml with a solution of hexanelethanol (92.5:7.5). The unreacted amine was converted to its corresponding butyramide by the addition of 10 μl of butyric anhydride. Each sample was analysed by chiral phase HPLC as described in Example 3. The results are summarised in Table 6.

TABLE 6 Effect of structural changes to the alcohol component of the acyl donor on the enantiospecificity of the resolution of 1-(1-naphthyl)ethylamine in dimethoxyethane by Chirazyme L2 lipase. E.e. of E.e. of product Time Conversion unreacted acetamide Enantiomeric Acyl donor (h) (%) amine (%) (%) ratio (E) Methyl 168 26 276 77 10 acetate Ethyl 168 37 53 91 33 acetate n-Propyl 168 34 47 92 35 acetate n-Butyl 168 36 52 94 43 acetate n-amyl 168 22 26 93 32 acetate Isopropyl 168 44 78 >98 650 acetate Isobutyl 168 40 64 95 95 acetate Isoamyl 168 35 51 94 61 acetate

EXAMPLE 7

Racemic 1,2,3,4-tetrahydro-1-naphthylamine (100 mg) was added to a solution of 4 ml of dimethoxyethane and 1 ml of acyl donor (see Table 7) and incubated at ambient temperature in the presence of 20 mg of Chirayme L2 (ummobilised Candida antarctica lipase). At intervals 0.5 ml samples were removed and diluted to 1 ml with dichloromethane. The unreacted amine was converted to its corresponding butyramide by the addition of 10 μl of butyric anhydride. Each sample was analysed by chiral phase GC as described in Example 4. The results are summarised in Table 7.

TABLE 7 Effect of structural changes to the alcohol component of the acyl donor on the enantiospecificity of the resolution of 1,2,3,4-tetrahydro- 1-naphthylamine in dimethoxyethane by Chirazyme L2 lipase. E.e. of E.e. of product Time Conversion unreacted acetamide Enantiomeric Acyl donor (h) (%) amine (%) (%) ratio (E) Methyl 120 41 49 69 9 acetate Ethyl 120 47 85 95 129 acetate n-Propyl 120 47 83 94 79 acetate n-Butyl 120 42 68 95 65 acetate n-Amyl 120 34 46 90 28 acetate Isopropyl 120 50 96 98 194 acetate Isobutyl 120 50 97 96 278 acetate Isoamyl 120 42 69 96 86 acetate

EXAMPLE 8

Racemic 2-amino-3,3-dimethylbutane (100 mg) was added to a solution of 4 ml of dimethoxyethane and 1 ml of acyl donor (see Table 8) and incubated at ambient temperature in the presence of 40 mg of Chirazyme L2 Cimmobilised Candida antarctica lipase). At intervals 0.5 ml samples were removed and diluted to 1 ml with dichloromethane. The unreacted amine was converted to its corresponding butyramide by the addition of 10 μl of butyric anhydride. Each sample was analysed by chiral phase GC as described in Example 5. The results are summansed in Table 8.

TABLE 8 Effect of structural changes to the alcohol component of the acyl donor on the enantiospecificity of the resolution of 2-amino-3,3-dimethylbutane in dimethoxyethane by Chirazyme L2 lipase. E.e. of E.e. of product Time Conversion unreacted acetamide Enantiomeric Acyl donor (h) (%) amine (%) (%) ratio (E) Methyl 336 22 17 59 5 acetate Ethyl 336 29 37 90 29 acetate n-Propyl 336 27 34 91 33 acetate n-Butyl 336 25 30 91 26 acetate n-Amyl 336 17 18 86 19 acetate Isopropyl 336 36 56 >98 791 acetate Isobutyl 336 30 41 96 68 acetate Isoamyl 336 27 35 94 51 acetate

EXAMPLE 9

Racemic 1-(1-naphthyoethylamine (100 mg) was added to 5 ml of acyl donor (see Table 9) and incubated at ambient temperature in the presence of 50 mg of Chirazyme L6 (Pseudomonas species lipase). At intervals 0.2 ml samples were removed and diluted to 1 ml with a solution of hexanelethanol (92.5:7.5). The unreacted amine was converted to its corresponding butyramide by the addition of 4 μl of butyric anhydride. Each sample was analysed by chiral phase HPLC as described in Example 3. The results are summarsed in Table 9.

TABLE 9 Effect of structural changes to the alcohol component of the acyl donor on the Enantiospecificity of the resolution of 1-(1-naphthyl)ethylamine by Chirazyme L6 lipase. E.e. of E.e. of product Time Conversion unreacted acetamide Enantiomeric Acyl donor (h) (%) amine (%) (%) ratio (E) Ethyl 266 21 12 47 3 acetate Isopropyl 266 12 12 87 18 acetate

EXAMPLE 10

Racemic 1,2,3,4-tetrahydro-1-naphthylamine (100 mg) was added to 5 ml of acyl donor (see Table 10) and incubated at ambient temperature in the presence of 50 mg of Chirazyme L6 (Pseudomonas species lipase). At intervals 0.2 ml samples were removed and diluted to 1 μl with MTBE. The unreacted amine was converted to its corresponding butyramide by the addition of 6 ml of butyric anhydride. Each sample was analysed by chiral phase GC as described in Example 4. The results are summarised in Table 10.

TABLE 10 Effect of structural changes to the alcohol component of the acyl donor on the Enantiospecificity of the resolution of 1,2,3,4-tetrahydro- 1-naphthylamine by Chirazyme L6 lipase. E.e. of E.e. of product Time Conversion unreacted acetamide Enantiomeric Acyl donor (h) (%) amine (%) (%) ratio (E) Ethyl 244 43 18 24 2 acetate Isopropyl 244 36 50 90 28 acetate Chirazyme is a trade mark of Boehringer Mannheim GmbH Chiralcel is a trade mark of Daicel Chemical Industries Limited 

What is claimed is:
 1. A process of resolution of chiral amines which comprises selectively reacting an enantiomer of the amine with an alkyl ester in the presence of a enantioselective lipase enzyme to produce an amide of that enantiomer and separating it from an unreacted enantiomer optionally after further reaction characterised in that the acid component of the ester has 1 to 10 carbon atoms and the parent acid is of formula RCOOH in which R is a hydrocarbyl group and the alkyl group of the ester is an isoalkyl group.
 2. A process as claimed in claim 1 in which the isoalkyl group is an isobutyl or isopropyl group.
 3. A process as claimed in claim 1 or 2 in which the unreacted enantiomer is recovered as such.
 4. A process as claimed in claims 1 or 2 in which the reacted enantiomer is converted to the original amine enantiomer by hydrolysis.
 5. A process as claimed in claim 1 or 2 in which the reacted enantiomer is utilised in its amide form.
 6. A process as claimed in claim 1 or claim 2 in which the hydrocarbyl group R is an unsubstituted alkyl group having 1 to 4 carbon atoms.
 7. A process as claimed in claim 1 or claim 2 in which 10 to 50% by weight of lipase is present based on the amine.
 8. A process as claimed in claim 1 or claim 2 in which the lipase is supported on a solid support.
 9. A process as claimed in claim 1 or claim 2 which is carried out in the presence of a solvent which is an ester, ether or hydrocarbon or a halogenated hydrocarbon which is free from OH and NH₂ groups.
 10. A process as claimed in claim 1 or claim 2 which is carried out at a temperature of 20 to 60° C.
 11. A process as claimed in claim 1 or claim 2 in which the amine has the formula

in which R¹ and R ² are, independently, an alkyl, cycloalkyl, alkenyl, alkynyl, or aryl group, said group being unsubstituted or substituted with a substituent selected from the group consisting of NO₂, SO₃H, COOR⁴, Cl, Br, F, I, OH, SO, SO₂, CN, alkoxy and, in the case of aryl groups, NH₂ in which R¹ and R²are different; R³ is H, an alkyl, cycloalkyl, alkenyl, alkynyl, or aryl group, said group being unsubstituted or substituted with a substituent selected from the group consisting of NO₂, SO₃H, COOR⁴, Cl, Br, F, I, OH, SO, SO₂, CN and alkoxy; and R⁴ is an alkyl, cycloalkyl, alkenyl, alkynyl or aryl group optionally substituted by one or more NO₂, SO₃H, COOR³, Cl, Br, F, I, OH, SO, SO₂, CN or alkoxy groups.
 12. A process as claimed in claim 11 in which the amine has the formula

in which R⁴ is an alkyl group having from 1 to 12 carbon atoms and R⁵ is an aryl, alkyl or cycloalkyl group, R⁵ being optionally substituted by one or more substituents selected from the group consisting of alkoxy, hydroxy, halogen, cyano, and when R⁵ is aryl, amine groups.
 13. A process according to claim 12 in which R⁴ is an alkyl group having from 1 to 6 carbon atoms.
 14. A process according to claim 12 in which R⁵ is either unsubstituted or substituted by substituents having at most 6 carbons in total in all of the said substituents.
 15. A process according to claim 12 in which the hydrocarbyl group R is an unsubstituted alkyl group having 1 to 4 carbon atoms.
 16. A process according to claim 15 in which R⁵ is unsubstituted.
 17. A process according to claim 16 in which R⁴ is an alkyl group having from 1 to 6 carbon atoms. 